Add multi-expansion support with data-driven protocol layer

Replace hardcoded WotLK protocol constants with a data-driven architecture
supporting Classic 1.12.1, TBC 2.4.3, and WotLK 3.3.5a. Each expansion
has JSON profiles for opcodes, update fields, and DBC layouts, plus C++
polymorphic packet parsers for binary format differences (movement flags,
speed fields, transport data, spline format, char enum layout).

Key components:
- ExpansionRegistry: scans Data/expansions/*/expansion.json at startup
- OpcodeTable: logical enum <-> wire values loaded from JSON
- UpdateFieldTable: field indices loaded from JSON per expansion
- DBCLayout: schema-driven DBC field lookups replacing magic numbers
- PacketParsers: WotLK/TBC/Classic parsers with correct flag positions
- Multi-manifest AssetManager: layered manifests with priority ordering
- HDPackManager: overlay texture packs with expansion compatibility
- Auth screen expansion picker replacing hardcoded version dropdown

include/pipeline/dbc_layout.hpp

@@ -0,0 +1,64 @@
+#pragma once
+
+#include <cstdint>
+#include <string>
+#include <unordered_map>
+
+namespace wowee {
+namespace pipeline {
+
+/**
+ * Maps DBC field names to column indices for a single DBC file.
+ * Column indices vary between WoW expansions.
+ */
+struct DBCFieldMap {
+    std::unordered_map<std::string, uint32_t> fields;
+
+    /** Get column index by field name. Returns 0xFFFFFFFF if unknown. */
+    uint32_t field(const std::string& name) const {
+        auto it = fields.find(name);
+        return (it != fields.end()) ? it->second : 0xFFFFFFFF;
+    }
+
+    /** Convenience operator for shorter syntax: layout["Name"] */
+    uint32_t operator[](const std::string& name) const { return field(name); }
+};
+
+/**
+ * Maps DBC file names to their field layouts.
+ * Loaded from JSON (e.g. Data/expansions/wotlk/dbc_layouts.json).
+ */
+class DBCLayout {
+public:
+    /** Load from JSON file. Returns true if successful. */
+    bool loadFromJson(const std::string& path);
+
+    /** Load built-in WotLK 3.3.5a defaults. */
+    void loadWotlkDefaults();
+
+    /** Get the field map for a DBC file. Returns nullptr if unknown. */
+    const DBCFieldMap* getLayout(const std::string& dbcName) const;
+
+    /** Number of DBC layouts loaded. */
+    size_t size() const { return layouts_.size(); }
+
+private:
+    std::unordered_map<std::string, DBCFieldMap> layouts_;
+};
+
+/**
+ * Global active DBC layout (set by Application at startup).
+ */
+void setActiveDBCLayout(const DBCLayout* layout);
+const DBCLayout* getActiveDBCLayout();
+
+/** Convenience: get field index for a DBC field. */
+inline uint32_t dbcField(const std::string& dbcName, const std::string& fieldName) {
+    const auto* l = getActiveDBCLayout();
+    if (!l) return 0xFFFFFFFF;
+    const auto* fm = l->getLayout(dbcName);
+    return fm ? fm->field(fieldName) : 0xFFFFFFFF;
+}
+
+} // namespace pipeline
+} // namespace wowee